The present invention relates to a system for generating a clock signal used when recording data on an optical disc, and more particularly to a generating system wherein the clock signal is extracted from wobble signal which is generated when a wobbled recording groove tracks formed on the disc is read out.
There has been known a Compact Disc-Recordable (CD-R) disc which is a write-once optical disc. The CD-R disc has a plurality of recording groove tracks in which data are recorded. The groove tracks are slightly wobbled in the radial direction of the disc by a wobble signal which is modulated with a frequency modulation. Therefore, when the CD-R disc is rotated, there is generated the wobble signal having a predetermined frequency (22.05 KHz). The carrier wave carries prerecorded information such as position information of the tracks.
In order to generate the wobble signal, a light beam is radiated on the groove tracks. The reflected beam is applied to a photodetector divided into two portions by a line optically parallel with a tangent of the groove tracks. A difference signal dependent on the difference between the outputs of the two portions is then applied to a band pass filter (BPF) having a transmitting range, the intermediate frequency of which corresponds to the predetermined frequency, that is 22.05 KHz. The rotation of the disc is controlled so that the average frequency of the extracted wobble signal becomes 22.05 KHz.
The extracted wobble signal is further used as a reference signal for producing a clock signal used at the recording. Namely, the wobble signal is a continuous signal having a frequency component which is synchronous with the rotation of the disc. When the phase of the clock signal is synchronous with that of the continuous signal, the clock signal accurately synchronizes with the rotation of the disc.
Meanwhile, the research and development of a write-once Digital Versatile Disc-Recordable (DVD-R) are much in progress. The recording capacity of the DVD disc is increased about 7 times as much as that of the CD-R disc. In order to increase the recording density, the distance between tracks of the DVD-R disc is decreased to substantially half of that of the CD-R disc. As a result, a part of the light beam applied on one groove track radiates the adjacent groove track, thereby causing a so-called crosstalk which cannot be ignored. When there occurs the crosstalk caused by the adjacent right and left groove tracks, the extracted wobble signal is interfered by the wobble component of the adjacent groove tracks so that the amplitude and the phase thereof deviate. In particular, when the phase is deviated, jitters inevitably occur, so that a clock signal which is accurately synchronous with the rotation of the disc cannot be generated.
More particularly, the crosstalk causes a deviation in a disc rotation control signal because of a phase difference between the reference clock signal and the extracted wobble signal. Accordingly, there occurs a difference in the period of time necessary for the rotation controlling operation dependent on the deviated rotation control signal to be effected, and a period of time it takes a phase-locked loop to generate the clock signal. Due to the inertia of the disc, it usually takes much longer time for the rotation control system to respond than the phase-locked loop. As a result, the phase of the clock signal is deviated from the phase of the rotation of the disc. Such a deviation causes the recording pits to be inaccurately formed in the groove track at recording as regards to the length thereof. Therefore, error occurs at reproduction of the disc.